1. Field of the Invention
The invention relates to the art of pusle generating circuits and more particularly concerns an electronic system including digital circuits with finite steps to generate frequency modulated pulses which vary in frequency randomly according to a prescribed statiscal distribution pattern or scheme.
2. State of the Prior Art
In radar simulation systems it is known to provide signals which simulate chaff, i.e. large masses of scattered particles which reflect radar beams. Since such chaff floats in air streams, it is subject to variations in wind speeds and in wind shear. Furthermore, there is a Doppler effect due to variations in distance between the instantaneous location of the chaff and the detaching radar, which effect is manifested by increases or decreases in the frequency of the reflected radar beams. If the radar simulation of the chaff is to be usable and realistic, the frequency of the simulated reflected radar signal must be variable to represent the changing Doppler effect and the effects of changing wind speed and wind shear.
An analogous situation exists in analyzing problems in engineering, behavioral sciences, social sciences, economics and other fields, where it is necessary to simulate a random statistical variation in behavior of some condition. Stated another way, where any condition is subject to modification by noise, it is desirable to be able to simulate the condition subject to a prescribed random frequency distribution scheme which simulates the effect of the noise.
Heretofore, the only way that frequency or phase modulated pulses could be varied according to a prescribed statistical distribution pattern for condition simulation purposes, was to create a computer program embodied in software in which data representing the random perturbations desired would be detailed. This method presented a number of very serious difficulties. In the first place, a very large capacity computer such as one having main frame capabilities, would be required to process the data. Secondly, the method was not adaptable to miniaturized, low cost, realtime operation equipment. Thirdly, there is an infinitely large number of theoretically possible statistical frequency distribution patterns, including among others: Gaussian type, i.e. bell curves, Rayleigh type distributions, logarithmic distributions, infinitely variable frequency distributions, and many more; so that it has not been practical to provide a complete computer program embodied in software detailing each desired series of digital pusles varying randomly according to a prescribed statistical frequency density pattern of perturbations. Furthermore, it was not possible to obtain such randomly perturbated signals in analog format if required. In any event there has not heretofore been available miniature, low cost, light weight, digital or digital analog equipment producing in real time the described randomly varying signals.
It has been proposed in prior U.S. Pat. No. 4,560,961 to provide a system for converting a digital chirp history to analog format by employing digital-analog circuits with finite steps to generate continuous frequency modulated analog signals corresponding in frequency and phase to the phase history of the chirps or frequency sweeps. However, in this patented system there is no provision for perturbating the digital signals in accordance with a predetermined statistical frequency density distribution pattern.
3. Summary of the Invention
It is a principal object of the present invention to provide a system having discrete components which create in real time the desired signal simulations, i.e. those involving signal frequency variations which are perturbated randomly according to a prescribed statistical distribution pattern. According to the invention first and second series of pusle groups in digital number format are supplied to the system. The two series of digital pulse groups represent first and second signals having different but constant or nearly constant frequencies. The first signal is applied to one input of a first object adder. The second signal is applied to a statistical frequency density distributor to generate digital pusles having statistically random frequency density perturbations of the second signal. Thus, the second signal is converted into a noise signal having a frequency which varies constantly according to any predetermined statistical pattern of random frequency density distribution, such as Gaussian (bell curve), Rayleigh, lognormal, infinitely variable, etc.
The perturbated noise digital signal is applied to another input of the first adder. The digital output of the first adder represents the first signal input retaining an average frequency value but perturbated in frequency by the noise signals. The perturbated signals are fed to a digital integrator consisting of a second adder connected to a register with pulse feedback to the second adder. The digital integrator changes the frequency variations of the perturbated signals to phase changes. The output of the digital integrator contains digital sine and cosine wave components of the perturbated signals. The digital sine and cosine signal components which are in phase quadrature with respect to each other are applied to digital-to-analog converters to produce analog sine and cosine signal outputs. These analog cosine (in phase) signals and sine (in phase quadrature) signals are then separately frequency modulated on a radio frequency carrier. The modulated signals are then combined in an analog adder to produce a frequency modulated RF carrier whose frequency varies randomly in conformance with the perturbations originally imposed on the first signal.